A conventional bicycle rack “A” attached to a rear end of a vehicle is shown in FIG. 1 and generally includes a vertical tube 10 which has one end connected to the vehicle and a horizontal section is connected to the other end of the vertical tube 10. A lower member 11 is fixed on the horizontal section and includes a plurality of recesses 110 defined in a top thereof. An upper member 12 is mounted on the lower member 11 and includes a plurality of recesses defined in an underside thereof, the recesses of the upper member 12 are cooperated with the recesses 110 of the lower member 11 so as to clamp bicycle frames (not shown) between the upper and lower members 12, 11. A plurality of bolts 13 as shown in FIG. 2 extend through the upper and lower members 12, 11 and are connected to the horizontal section to secure the bicycle frames by teeth 132 on the bolts 13. However, the heads 131 of the bolts 13 can be easily unscrewed by any one without permission so that the bicycles on the bicycle rack “A” might be stolen by unscrewing the bolts 13.
The present invention intends to provide a locking head for a bolt assembly wherein the head of the bolt can be set to be freely rotatable relative to the shank of the bolt such that the bolt cannot be unscrewed.